Shipboard fluid control systems are subjected to a severe corrosive environment. In the particular case of offshore drill ships fluid control systems are incorporated in the support structure for the riser pipe, a typical riser extending five hundred to three thousand feet beneath the ship and supported by riser cables extending downwardly from the ship. As the ship rises and falls on the ocean swells the support structure must be free to extend and contract smoothly. Typically a large hydraulic cylinder (18 inches in diameter, 16 feet of piston travel) connects each riser cable to the ship. A hydraulic circuit connects the cylinder to a gas pressurized accumlator. The weight of the riser results in the storage of sufficient potential energy in the cable and fluid control system to cause heavy damage to the ship and hazard to personnel upon cable breakage or other accident.
Emergency flow shut off devices are provided in the fluid circuit to prevent destruction of the hydraulic cylinder and accumulator. The devices prevent movement of hydraulic fluid thereby locking the fluid circuit and retaining the potential energy stored therein for subsequent controlled release. Such prior are devices require that the drilling operation be suspended while the devices are tested. There is no assurance that the devices are always operable between tests. The seriousness of the hazard upon failure of a prior art device during an emergency has led to the features of the present invention as disclosed below.
Other flow shut off or retardation devices are exemplified by U.S. Pat. Nos. 3,760,840; 3,561,574; 3,106,992; 3,255,850; and 3,739,808. These devices are directed to the control of fluid systems having different properties from that or riser support structures but are illustrative of the prior art.